_Action._—A powerful muscle of expiration. In contracting, lessens the cubic content of the thorax. =The Diaphragm= (Fig. 47, No. 10; Fig. 33, No. 9). _Description._—The diaphragm in the domestic fowl is rudimentary. It consists of a thin semi-transparent membrane, situated between and separating the thoracic and the abdominal cavities. It readily conforms to the various organs pressing upon it from each side. On each side are three rudimentary muscles which extend from the vertebral heads of the second, the third, and the fourth sternal ribs. The fibers of these muscles spread out in fan-like upon the diaphragm, and are seen just above the digitations of the triangularis sterni. THE ABDOMINAL MUSCLES Obliquus abdominis externus Obliquus abdominis internus Rectus abdominis Transversalis abdominis =Obliquus Abdominis Externus= (Fig. 22, No. 1). _Location._—The outermost muscle of the lateral abdominal wall. _Attachments._—By a delicate aponeurotic membrane, from the sides of all the true dorsal ribs, from the posterior border of the last vertebral rib and the adjoining margins of the pelvis, and from the entire posterior surface of the inferior border of the post-pubic element of the pelvis; by aponeurosis, blending with the fascia toward the root of the tail and lower part of the abdomen; and by aponeurosis to the sides of the sternum, to the under part of the pectoralis major muscle, and to the zyphoid prolongation. Also by aponeurosis it meets its fellow from the opposite side, at the linea alba. [Illustration: FIG. 22.—The abdominal muscles of a hen. Right side. 1, Obliquus abdominis externus. 2, Obliquus abdominis internus. 3, Peritoneum covering intestines. 4, Peritoneum covering the gizzard. 5, The lungs (note how small they are from a relative standpoint). 6, The great sciatic or ischiadic nerve. 7, Lumbar nerves. 8, Kidney. 9, Brachial plexus. 10, Pericardium of the base of the heart. 11, Anterior vena cava. 12, Oil gland. 13, Showing a sacculation of the abdominal muscles. There is no tunica abdominalis in the fowl. ] =Obliquus Abdominis Internus= (Fig. 22, No. 2). _Location._—Just internal with regard to the external oblique. _Attachments._—By aponeurosis, from the posterior third of the post-pubic element of the pelvis; by muscular fibers, from the balance of the bone; and by a few fibers, from the iliac border posterior to the acetabulum. Anteriorly, these fibers are inserted to the entire posterior margin of the last vertebral rib, and into the pleurapophysial head of the last costal rib. =Rectus Abdominis= (Fig. 13, No. 10 and Fig. 23, No. 1). _Location._—The inferior median abdominal wall.